Most CO detectors are pretty reliable, I think the best method is to have some redundancy,And location is critical, and to keep them clean! The best detector are not worth anything if dust get into the sensor.

Hmmm. I don't know about best or most expensive but if you have two or three of them its likely they all won't fail at once. I use the Kiddie NightHawk with digital PPM readout. One on each floor. They work great .. I've seen readings on them when I had draft failures in the past.

Gary1,a lot of us have the Kiddy but anyone with a readout will be good.I like to see if you have CO because the alarm doesn't sound with lower levels(under 70ppm)but it will read the lower levels(30ppm or higher)I have 5 plug in with battery back up and 1 hardwired into my alarm system.